On-board generators in aircraft provide power to electrical systems of the aircraft generally only when the engines are running. However, there are times and places when running the aircraft engines is not feasible to power the aircraft's electrical systems. Accordingly, ground power units have been developed to provide power to aircraft electrical systems by either connecting to the existing power grid or by utilizing a gas or diesel engine with a generator. Unfortunately, not all aircraft use power having the same characteristics. Common power standards of aircraft electrical systems are 115 V 400 Hz 3-phase AC and 270 V DC, which is obtained by a direct rectification of 115 V AC power.
However, the maintenance of two ground power units for supplying 115 V 400 Hz AC and 270 V DC has obvious disadvantages. Accordingly, it would be desirable to have a ground power unit to provide both 115 V 400 Hz AC power and 270 V DC.